(1) Field of the Invention
The present invention relates generally to the preparation of textile fibers, and more particularly to the preparation of fibers containing an intrinsically conductive polymer (ICP) by solution spinning.
(2) Description of Related Art
Several ICP's, one notable example of which is polyaniline, are recognized as being promising for applications which require the properties of a polymer, but would benefit from enhanced electrical or electromagnetic properties. Nevertheless, the use of polyaniline in its protonated, or conductive form, has been limited because it has been considered difficult to process due to low solubility in normal commercial solvents. The low solubility of polyaniline compositions have precluded their use in producing fibers by solution spinning or wet spinning because standard wet spinning methods require a polymer concentration of 15 to 20 percent in the spinning solution.
Recently, new methods for the preparation of fibers containing conductive forms of polyaniline have been reported. Methods to coat fibers by electrochemically forming a conductive organic polymer on the outer surface of a polymeric fiber were reported in U.S. Pat. No. 5,423,956. Similarly, polyaniline with a counterion doping agent has been polymerized onto the surface of a fiber or fabric material. (See U.S. Pat. No. 4,803,096). These and other processes that polymerize polyaniline on the surface of fibers or textiles have the drawbacks of requiring additional manufacturing steps and result in the ICP being limited to the surface of the fiber rather than distributed throughout the fiber cross-section.
Another approach described by Hsu in U.S. Pat. No. 5,248,554, impregnates filaments of p-aramid yarns with polyaniline by passing the yarn through a solution of polyaniline in concentrated sulfuric acid. The acid causes the fiber to swell and crack longitudinally allowing the polyaniline to penetrate into the fiber. Although the method results in the penetration of polyaniline into the fiber interior, the process results in loss of strength of the fiber and can not provide a fiber containing polyaniline doped with larger organic acids since the larger acids can not diffuse into the fiber after the polyaniline impregnation.
Andreatta and coworkers report a method of producing polyaniline fibers from a solution in concentrated sulfuric acid (Andreatta et al., Synth. Metals, 26:383-389, 1988) and Epstein and Yue report spinning fibers of sulfonated polyaniline from solutions in sulfuric acid or sodium hydroxide (U.S. Pat. No. 5,135,696). However, fibers composed entirely of polyaniline are brittle and inflexible and unable to withstand the wear and tear of textile use. Neither of these groups disclosed the spinning of fibers from polymer blends containing polyaniline, presumably because polymers typically used in textile fibers are either insoluble or unstable in the acids or caustic solutions required by the method.
Smith et al., U.S. Pat. No. 5,470,505, also reported spinning polyaniline fibers having high conductivity from a concentrated sulfuric acid solution into a bath of chilled water. The group used the same method to spin fibers from a 1:1 mixture of polyaniline and poly-para(phenylene terephthalamide) (Kevlar.RTM.), but reported no blends with more commonly used fiber-forming polymers and disclosed no tensile properties for the resulting fibers. Furthermore, the use of organic acid salts of an ICP was not taught, presumably because the sulfuric acid would have replaced the organic acid dopant during the spinning process.
High molecular weight polyaniline has also been spun into fibers from the non-conductive form of polyaniline dissolved in N-methyl pyrrolidone followed by subsequent doping of the fibers with HCl to produce the conductive form of polyaniline (See, for example, U.S. Pat. Nos. 5,177,187, 5,258,472 and 5,312,686 to MacDiarmid et al.). However, a solution of polyaniline in NMP is unstable and gels rapidly at room temperature. Although Han, U.S. Pat. No. 5,171,478, reported spinning fibers of neutral polyaniline in NMP from a "blue solid rubber-like gel", followed by redoping in para-toluenesulfonic acid, it is doubtful that such high viscosity materials could be easily handled in commercial scale processes.
Tzou, K. T. and R. V. Gregory, Synth. Metals, 69:109-112, 1995, report that a solution of neutral, undoped polyaniline in N,N'-dimethyl propylene urea is more stable than NMP as a spinning solvent. Also, Cohen et al., EPO 446,943 A2, 1991, spun polyaniline fibers from concentrated solutions (20%) using solvents such as 1,4 diaminocyclohexane and 1,5-diazabicyclo(4,3,0)-non-5-ene, but such solutions are very sensitive to shear rates applied during mixing. Furthermore, as noted above, fibers produced from polyaniline alone are unsuitable for most textile applications. Also, post-spinning doping to increase polyaniline conductivity results in doping only the surface of the fibers and usually requires that small dopant molecules (e.g., HCl) be used so that doping time will not be prohibitively long. But these small dopants easily diffuse out of a fiber during washing, for example, leaving it undoped. Also, such doping requires a separate and additional processing step which increases the complexity and cost of manufacture. Dopants of larger molecular size, such as certain organic acids, would have the advantage of being less prone to diffuse out of the doped fiber, but, conversely, could not be added to the fiber after its formation without unreasonably long diffusion time or partial destruction of the fiber structure.
In another approach, polyaniline with a counterion doping agent has been polymerized onto the surface of a fiber or fabric material (See U.S. Pat. No. 4,803,096 to Kuhn et al.). But this method also results in the dopant being restricted to the surface of the fibers. Furthermore, achieving an adequate adhesion of a surface coating of an ICP to the host fiber can be a problem.
Cao et al., Synth. Metals, 48:91-97, 1992, cast films of doped polyaniline blended with polymers such as polymethylmethacrylate and polyethylene, but do not disclose how fibers may be formed by conventional fiber spinning techniques. This is understandable inasmuch as the technology used for film formation is different than that used for fiber spinning and systems of polymers and solvents useful for film forming may not be useful at all for fiber spinning.
Thus it would be desirable to provide fibers which contain an ICP, such as polyaniline, doped with an organic acid and in its electrically conductive form, that is dispersed in the fiber and which also possess the mechanical properties which permit their successful use in textile materials. It would additionally be desirable to provide an improved method for producing such fibers.